Clarification tanks (aka settlement tanks) are used in a variety of applications, including removal of solids from sewage and removal of drill tailings and heavier-than water compounds present in drilling fluids to permit recycled use of water that may be present in such drilling fluids.
A common design for fluid settling systems is the use of rectangular vessels that are segmented into a series of smaller compartments by dividers or weirs that restrict the forward movement of the fluids being circulated. Areas of reduced fluid velocity conducive to the settling of solids, or settlement compartments, are thereby created. The physical shape of such rectangular settling compartments present a number of inefficiencies. In particular, the right angle corners of rectangular shaped compartments becomes an effective trapping mechanism where fluid velocity is adversely affected creating an area prone to solids deposition. The deposition of the solids in these corners effectively reduces the working volume of the chamber or compartment and thus reduces the overall efficiency of the entire settling tank. Furthermore, as these solids collect, the weight of successive solids collecting on top of other solids compresses these settled solids making their removal, labor and time intensive. Further compounding the ineffectiveness of the rectangular compartments is the addition of suction lines and other internal plumbing that also creates traps, hampering the effectiveness of the settling.
CA 2,485,875 entitled “Settling Tank and Method for Separating a Solids-Containing Fluid” and granted to the within inventor, teaches a generally rectangular settling tank, having an inlet and an upper outlet at opposite ends of an individual tank so that the fluid passes generally in a first direction from the inlet toward the upper outlet. A backwash fluid source, typically a nozzle, for directing backwash fluid in a second direction which is substantially opposite such first direction, is provided. It is believed the opposite directions 182,192 of flow of the solids-containing fluid 162 and the backwash fluid 190 provides or causes a rolling action or rolling fluid flow, indicated by arrow 194 in FIG. 16 therof, and may also increase the residence time of the solids-containing fluid 162 within the settling compartment 168, which may enhance the settling of the solids therein. The rectangular shape of the design and the presence of internal plumbing, however, creates the potential for traps and solids collection.
All clarifying/settlement tanks, including rectangular systems, used for separating solids from solids-containing liquids, will typically have the problem, after operation of a period of time, of build-up of precipitated solids on the bottom of the tank. Such build-up, if permitted to continue, detrimentally reduces the volume of the settling tank and thus the tank's speed and capacity to clarify and separate solids from quantities of solids-containing liquids. The solids need be removed from the tank, and transported to an area of storage and concentration, so operation of the clarification tank may continue.
One means/method of removing the solids from the tank is, of course, to cease the clarification operation and access the tank from the top to collect the solids and remove them from the tank. Detrimentally, however, clarification tanks typically run in continuous as opposed to batch mode, and continually separate solids from an incoming solids-containing stream. Thus having to stop such continuous operation for removal of solids from the bottom of the tank is not only time consuming and labour intensive, but further causes complete cessation of all upstream supply of solids-containing fluids until the precipitated solids have been removed from the tank and the tank brought back into operation. To deal with this problem additional bypass tanks are employed, and shut-downs for solids removal from each are scheduled in a “staggered” manner, to allow continual uninterrupted treatment of a solids-containing fluid being generated upstream. Such additional processing capacity, and operation of the units in a “staggered manner”, adds greater capital cost and expense.
An alternative known manner and apparatus for allowing removal of sediment from settlement tanks but allowing for continued clarification of fluids in the tanks, but which adds considerable cost and mechanical complexity, is to provide a frusto-conical base and a rotating mechanical arm or arms (“rakes”) which continually “sweeps” the frusto-conical interior surface of any settled sediment and continually directs such sediment downwardly into a solids outlet, situated centrally and co-axially within the frusto-conical surface.
EP Patent Application 0010395 is an example of one such sedimentation tank apparatus employing a rotating mechanical rake. Tank 10 of EP '395 comprises a cylindrical outer wall 11, a conical bottom wall 12, and a centrally located discharge sump outlet 14, as seen from FIG. 1 thereof. A motor drive mechanism 17 is provided for driving a central rotatable drive shaft 18 which is mounted to a rotary rake structure 20 for moving settled underflow to sump 14 on the frusto-conical bottom of tank 10.
EP '395 further teaches a submerged inlet feed structure 27 comprising a pair of superimposed upper and lower branches 28, 29 which lead tangentially into upper and lower fed channels 25, 26. In such manner the liquid influent is caused to flow inwardly in opposite directions from channels 25, 26 to shear in a plane along the entire length of the channels at twice the velocity. In the shear zone B the energy of the two steams is converted into random turbulence.
Along similar lines is U.S. Pat. No. 3,006,474, also naming the same inventor as EP '395, entitled “Method and Means for converting the Kinetic Energy of a Fluid Stream into Random Turbulence”, having rake arms 74,75 which deliver sludge over the tank bottom 49 into a conical sump 76 for withdrawal through discharge pipe 77.
U.S. Pat. No. 6,793,814 entitled “Clarifying Tank” provides a cylindrical tank, having a frusto-conical bottom, and a centrally-located solids outlet 150 therein. In one embodiment a conical auger 160 is provided, rotated about shaft 180, to compress solids in the bottom frusto-conical portion of the tank and move them toward solids outlet 150. In the embodiment shown in FIG. 3 fluid enters the tank through tangential fluid inlet 410 [situated above the frusto-conical portion (ref. FIG. 3)] creating a vortex (col. 5, lines 52) which tends to move solid particles within the fluid toward the wall of the tank, so that fluid which remains nearer the center of tank 310 will thus become relatively free of solid particles, and fluid outlet 440 is provided to permit removal of this substantially clean fluid from the center of tank 310. Tank 310 also includes a static coalescing spiral 500 having a series of flights 510 inclined upward and extending into the rotating fluid within the tank, and runs in the opposite direction of the flow of fluid. Static spiral 500 acts to coalesce smaller solid particles into larger particles that settle out of the fluid at increased rate.
U.S. Pat. No. 857,626 to a “Water Tank” teaches a cylindrical tank having a hemi-ellipsoid bottom 2. By the arrangement of a blow-off pipe 23 and a valve 24 in the bottom of the inlet pipe 15, all mud and other sediment in the water will settle in the bottom of the inlet pipe and by opening the valve 24, such mud may be readily blown out by the force of the water rushing through the valve, and it thereby becomes unnecessary to empty the entire tank to remove the sediment as in the case where the sediment is allowed to settle in the bottom of the tank, instead of in the bottom of the inlet pipe.
The foregoing background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information, or the reference in the drawings to “prior art” constitutes relevant prior art against the present invention.